#define _CRT_SECURE_NO_WARNINGS 1

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int sumOfLeftLeaves(TreeNode* root)
    {
        int sum = 0;
        LeftSum(root, nullptr, sum);
        return sum;
    }

private:
    void LeftSum(TreeNode* cur, TreeNode* parent, int& sum)
    {
        if (cur == nullptr) {
            return;
        }

        if (!cur->left && !cur->right && parent && parent->left == cur)
        {
            sum += cur->val;
        }

        LeftSum(cur->left, cur, sum);
        LeftSum(cur->right, cur, sum);
    }
};